Computing: Could a “virtual computer”, built from software, help to save today's digital documents for historians of the future?

WHEN future historians turn their attention to the early 21st century, electronic documents will be vital to their understanding of our times. Old web pages may not turn yellow and brittle like paper, but the digital documents of today's culture face a more serious threat: the disappearance of computers able to read them. Even a relatively simple electronic item, such as a picture, requires software to present it as a visible image, but 100 years from now, today's computers will have long since become obsolete. More complex items, like CD-ROMs or videos, will be unreadable even sooner.

In 1986, for example, 900 years after the Domesday book, the BBC launched a project to compile data about Britain, including maps, video and text. The results were recorded on laserdiscs that could only be read by a special system based around a BBC Micro home computer. But since the disks were unreadable on any other system, this pioneering example of multimedia was nearly lost for ever. It took two and a half years of patient work with one of the few surviving machines to move the data on to a modern PC (it can be seen online at www.domesday1986.com).

National libraries are just starting to grapple with this problem as part of their new mandate to preserve digital culture. “It is a major problem, but it is remarkable how little known it is,” says Hilde van Wijngaarden, head of digital preservation at the National Library of the Netherlands. “People just accept that things no longer work after ten years.”

Keeping working examples of all computer hardware is impractical, so the most popular preservation strategy is to copy files from one generation of hardware to the next. The problem is that today's word processors and web browsers, for example, do not always display files in the same way that older software did. An accumulation of subtle errors can eventually make the original item unreadable. An alternative approach, called emulation, uses software to simulate the old hardware on a modern computer, to allow old software to run. But today's emulators will need another emulator to run on the next generation of hardware, which will need another emulator for the next generation, and so on. This can also introduce errors.

So the National Library of the Netherlands is exploring a third option, using a simulated computer that exists only in software. It is called the Universal Virtual Computer (UVC) and is being developed by IBM, a computer giant. The researchers are writing programs to run on this virtual computer that decode different document formats. Future libraries will have to write software that emulates the virtual computer on each new generation of computer systems. But once that is done, they will be able to view all their stored documents using the decoders written for the virtual computer, which only have to be written once. “The decoder can be tested for correctness today, while the format is still readable,” says Raymond van Diessen of IBM.

His team has written decoders for two common image formats, JPEG and GIF. They plan to move on to Adobe's PDF format. IBM is also talking to drug firms, which are required to store data from clinical trials for long periods. Ultimately, the aim is to be able to preserve anything from simple web pages to complex data sets. Ominously, some scientific data from the 1970s has already crumbled into unreadable digital bits.